The present invention relates to phantoms for nuclear imaging, and more specifically to a phantom fillable with a solution of a radioisotope for providing radioactive regions or xe2x80x9chot spotsxe2x80x9d within a less radioactive or xe2x80x9cwarmxe2x80x9d background.
Medical physicists and researchers in nuclear imaging commonly use fillable phantoms for characterizing the imaging capabilities of both single photon emission computed tomography (SPECT) and positron emission tomography (PET) systems. Most common phantoms have simple designs and often are designed to measure specific imaging parameters, such as line sources for spatial resolution and open fillable chambers for uniformity. However, in recent years, there has been growing interest in more complex phantom designs to assess imaging performance in realistic imaging situations.
With the more widespread use of radiopharmaceuticals such as 18F-fluorodeoxyglucose (FDG), 99mTc-Sestamibi, and 111In-labeled and 131I-labeled monoclonal antibodies for oncology imaging, the ability of SPECT and PET scanners to detect lesions of higher activity concentration with respect to the surrounding tissue is of high interest. To simulate the imaging of tumors in patients, phantoms with fillable spheres are typically used. Before imaging the phantom, the physicist or technologist fills the spheres and background chamber with solutions with the desired concentration ratio of activity. Although the design of this phantom is relatively simple, there are several disadvantages to this approach. The accuracy and reproducibility of the spheres-to-background concentration ratio is not guaranteed, since the steps of measuring the activities and volumes and of filling the chambers have the possibility of error. In addition, such phantoms with multiple fillable chambers are inconvenient to use because of the number of steps involved in preparation. The time required to fill these phantoms (typically 30 minutes) is costly, and it also prevents lesion detectability studies with isotopes with very short half-lives. A more convenient and more reproducible phantom design that simulates active lesions in a patient is therefore highly desirable.
Attenuation correction in nuclear medicine also is rising in significance. Attenuation correction provides a more quantitative uptake distribution in images, and many believe that more accurate diagnosis can be obtained. The commercial implementations of attenuation correction in nuclear imaging systems are many and often fundamentally different. Commercial attenuation correction approaches range from stationary line sources, to scanning point sources, to multi-modality x-ray computed tomography (CT) systems: CT/SPECT and CT/PET. Comparing the imaging capabilities between scanners and ensuring the daily quality of attenuation-corrected images is vital. Other than xe2x80x9ccold-spotxe2x80x9d phantoms using inserts of different materials, there are no phantoms specifically designed to test the attenuation correction capability of nuclear medicine systems, even though the need for such a phantom is growing.
Anthropomorphic phantom designs are also of interest. Phantoms with inserts to simulate cardiac uptake are commercially available, and anthropomorphic phantoms with chambers for lungs, heart, and liver are also available. Such phantoms are useful for better simulating patient imaging and for observing the effect of attenuation correction and scatter correction. While these complex phantoms provide more imaging detail, they are correspondingly more inconvenient to use because of the increased number of chambers to fill.
Phantoms are routinely used in nuclear medicine for several purposes. Phantoms for single photon emission computed tomography (SPECT) and positron emission tomography (PET) are fixtures that contain a radioisotope source of a specific geometry. Often, phantoms are used for characterizing the performance of SPECT and PET systems. There are guidelines from organizations such as the National Electrical Manufacturers Association (NEMA) and the American Association of Physicists in Medicine (AAPM) which recommend several phantoms, such as point sources, line sources, and fillable cylindrical chambers. These phantoms are specially designed to measure specific performance characteristics of the scanner, including spatial resolution and sensitivity. Manufacturers also use these performance measures in marketing their SPECT and PET systems.
An important use for phantoms is for quality assurance (QA) of SPECT and PET scanners. Hospital physicists follow specific daily, weekly, and monthly protocols to ensure proper operation of their imaging systems. Regular QA is critical for ensuring proper image quality and proper diagnostic accuracy of patient images. While the methodology for performance testing from NEMA gives valuable information about system performance, many of the tests require specialized equipment and sophisticated software. As a result, more convenient QA phantoms are used for routine testing. The most desirable characteristics of phantoms for QA are ease of use and reproducibility. Therefore, QA phantoms are designed to be simple and convenient to fill.
For example, as shown in FIG. 1A, the Jaszczak phantom 10 comprises a cylindrical chamber 12 containing arrays of solid plastic rods 14 and several solid plastic spheres 16 of various diameters. The Jaszczak phantom is what is commonly referred to as a xe2x80x9ccold spotxe2x80x9d phantom having a xe2x80x9cwarm background,xe2x80x9d in that the spheres show up on a scan as radio-neutral regions in a radioactive background. The Jaszczak phantom 10 is pillable using a single injection of radioisotope, and the resulting images yield information regarding scanner contrast resolution and performance, but not lesion conspicuity.
Another field of interest for SPECT and PET phantoms is research. Physicists utilize phantoms of various geometries, and often simulate the human body in order to test novel image reconstruction algorithms and data correction capabilities. Examples of such phantoms are the anthropomorphic torso phantom 20 available from Data Spectrum, shown in FIG. 1B, and the Hoffman brain phantom 30, shown in FIG. 1C.
The torso phantom 20 comprises a plurality of individual chambers 22, representing human organs, which can be filled with a radioactive solution. A main chamber 24 can be filled with a different radioactive solution. Thus, the torso phantom 20 can be used to provide xe2x80x9chotxe2x80x9d spots in a xe2x80x9cwarmxe2x80x9d background. However, the steps of filing each of the individual chambers 22 of the torso phantom 20 is time consuming. Further, the requirement of the preparation of different radioactive solutions for producing contrast among the chambers 22 and/or between the chambers 22 and the main chamber 24 leads to poor repeatability, since human error will naturally produce variations in the concentrations of the solutions each time they are prepared.
The Hoffman brain phantom 30 comprises a single fillable chamber defined by nineteen individual plates 32 that stack within a cylindrical container. The plates 32 each include open chamber portions 34 which hold the radioactive solution when the phantom 30 is filled. When the plates 32 are stacked together, varying thicknesses of the chamber portions 34 and surrounding solid portions cooperate to simulate the gray and white matter of the human brain.
Nuclear medicine research using phantoms has been steadily increasing in recent years because of the availability of advanced imaging hardware and software. The efficacy of methods for quantitative SPECT and PET, such as attenuation correction, scatter correction, and collimator deblurring, continues to be investigated by researchers. Of particular interest is the effect of these corrections on specific clinical applications, such as for oncology and cardiac imaging.
Another approach to phantoms for nuclear imaging was recently developed in which an ink-jet printer loaded with radioisotope solution is used to print pages with the desired planar isotope distributions. To create a three-dimensional phantom having an activity distribution that is virtually attenuation-free and scatter-free, the pages are stacked vertically with a constant spacing (such as 10 mm) between the planes. Slabs of other material can be placed between the sheets to modify the attenuation and scatter properties. While this approach is useful in allowing greater flexibility in phantom design, it is not convenient for regular use, since a user must prepare the printer, print many sheets, cut the sheets to size, and stack the sheets. Improving phantom""s axial sampling requires more sheets and more preparation time. Thus a phantom design that is more convenient and that is able to simulate the complex imaging tasks of interest is still highly desirable.
Lesion detection is a specific imaging task of great interest in nuclear medicine. The percentage of oncology cases in nuclear medicine imaging has been steadily increasing in recent years and is now approximately 40% of the total. The more widespread use of radiopharmaceuticals, such as 18F-fluorodeoxyglucose (FDG), 99mTc-Sestamibi, and 111In-labeled and 131I-labeled monoclonal antibodies, has driven this increase in oncology cases. The measure of performance that is of clinical interest is the minimum detectable lesion size with respect to the lesion-to-background uptake ratio. The detectable lesion size is a key measure of both clinical conspicuity and relative scanner performance. For example, a recently published study by Coleman et al. used a torso phantom with fillable spheres to compare the conspicuity of lung tumors using three types of 511 keV imaging systems: dedicated PET, gamma camera PET, and collimated SPECT. Phantom studies such as this are critical for assessing the efficacy of SPECT and PET imaging for oncology (which enters into decisions on insurance reimbursement), since the presence or absence of secondary lesions is a strong consideration for the course of therapy to follow.
Another important factor in lesion detectability is the location of the lesion within the body. Because of the nonuniform response of sensitivity, resolution, attenuation, and scatter, the ability to detect a lesion depends greatly on the organs being imaged. Therefore, using phantoms that accurately mimic the human body is highly desirable, since the lesion detectability in a standard cylindrical phantom may be quite different. However, existing anthropomorphic phantoms have limited reproducibility and are relatively difficult to use.
Lesion detectability in SPECT and PET imaging has become such an important performance characteristic of scanners that lesion detection has recently been included in an industry standard test. The recently revised NEMA (National Electronic Manufacturers Association) NU-2-2001 standard xe2x80x9cPerformance Measurements of Positron Emission Tomographsxe2x80x9d includes an image quality measurement based upon multiple fillable spheres. Key issues in implementing this standard include reproducibility and accuracy.
Reproducibility of phantom imaging is a critical characteristic for comparing scanner performance and for routine quality assurance. Lesion detection studies require knowledge of the activity concentration ratio of the simulated lesions with respect to the background volume. Filling multiple chambers in a phantom introduces the possibility of producing an incorrect concentration ratio.
The reproducibility of the concentration ratio is limited with conventional phantom designs, such as the fillable spheres 40, 50 shown respectively in FIGS. 2A and 2B. In order to fill multiple chambers with the desired concentration ratios, the relative volumes of the chambers must be known precisely, and measuring the activities and volumes of liquids introduces many possibilities for error. In addition, as shown in FIG. 2A, leaving a bubble 42 in the fillable chamber 44 alters the effective concentration. As shown in FIG. 2B, it is also possible to xe2x80x9coverfillxe2x80x9d a sphere 50 by adding liquid to the neck 52 of the sphere 50 that attaches to a mounting rod 54. Another point that affects the imaging accuracy is the nonzero thickness of the walls 46, 56 of the fillable chambers. The walls 46, 56 translate into a region of zero activity, which also alters the effective concentration. The effect is substantial for small chambers. For example, a fillable sphere with outside diameter of 9 mm and a wall thickness of 0.5 mm has a fractional xe2x80x9cdead spacexe2x80x9d of 16%. The errors in the apparent activity concentration and size of fillable spheres in PET imaging have been found to be large for small spheroids, approaching 25% for spheres of 13 mm diameter, for example.
Finally, convenience of filling the phantom is a vital requirement for its widespread use. Unfortunately, for conventional phantom designs the difficulty of filling the phantom rises as the complexity of the phantom rises. The main obstacle is the number of chambers to be filled, since each chamber requires the steps of planning, measuring, and filling. Lack of convenience has been a main reason why lesion detection phantoms are not used for routine QA. The time required to prepare accurately a phantom with fillable spheres depends on the experience and patience of the technologist, but the typical time is on the order of 30 minutes or longer.
The present invention provides a fillable phantom for use with nuclear imaging systems. The phantom comprises a container comprising a connector for filling the container with radioactive solution, a porous medium within the container for holding the radioactive solution, and a contrasting region formed in the porous medium and being in fluid communication with the porous medium.
According to another aspect, the present invention provides a fillable phantom for use with nuclear imaging systems comprising a porous medium for holding a radioactive solution, and a contrasting region formed in the porous medium. The porous medium comprises a solid material comprising a plurality of vertical channels formed in the solid material.
According to a further aspect, the present invention provides a fillable phantom for use with nuclear imaging systems comprising a porous medium for holding a radioactive solution, and a contrasting region formed in the porous medium. The porous medium comprises a plurality pellets bonded together and defining interstices.
According to a still further aspect, the present invention provides a fillable phantom for use with nuclear imaging systems, the phantom comprising a porous medium for holding a radioactive solution, and a contrasting region formed in the porous medium. The porous medium comprises an open cell foam.
According to yet a further aspect, the present invention provides a system for filling a fillable phantom for use with nuclear imaging systems. The system comprises a phantom comprising a container, a porous medium within the container, and a contrasting region formed in and in fluid communication with the porous medium. The system further comprises an external mixing container for preparing a radioactive solution with which the phantom is to be filled. The chamber is connectable to the phantom for filling the phantom with the solution and for draining the solution from the phantom.
According to yet another aspect, the present invention provides a method of preparing a phantom for use with a nuclear imaging system. The method comprises steps of: providing a phantom comprising a phantom container and a porous medium within the container having a contrasting region formed in the porous medium, and filling the phantom with a radioactive solution so that the solution flows into the porous medium and into each of the contrasting regions. The radioactive solution comprises a liquid medium and a radioactive isotope.
According to a still further aspect, the present invention provides a process of manufacturing a phantom for use with a nuclear imaging system. The process comprises steps of: cutting a plurality of wafers; punching a plurality of perforations in each of the plurality of wafers; drilling holes in appropriate ones of the plurality of wafers so that when the wafers are stacked with the plurality of perforations aligned, a void of a desired shape is formed; stacking the wafers with the pluralities of perforations in alignment; bonding the stacked wafers together to form a porous medium for the phantom; and inserting the wafers into a phantom container.
According to an even further aspect, the present invention provides a process of manufacturing a phantom for use with a nuclear imaging system. The process comprises steps of: filling a container with a plurality of pellets; imbedding a dissolvable solid among the plurality of pellets; compressing the pellets within the container; bonding the plurality of pellets to form a porous medium; dissolving the dissolvable solid; and eliminating the dissolved solid from the porous medium to leave a void in the shape of the dissolvable solid.